1. Field of the Invention
This invention relates to a thermal recording apparatus and more particularly to a thermal recording apparatus in which a plurality of heating resistor elements are simultaneously energized to be heated so that information is recorded by coloring those portions of thermally responsive recording paper which are in contact with the heating resistor elements.
2. Description of the Prior Art
The conventional thermal recording apparatus usually has a plurality of heating resistor elements arranged in a line, the selected elements of which are energized for a predetermined period of time so as to color those portions of thermally responsive paper which are in contact with those selected elements. An example of such a thermal recording apparatus is disclosed in U.S Pat. No. 3,984,844 granted on Oct. 5, 1976 to K. Tanno et al. and assigned to the assignee of the present invention, entitled "THERMAL RECORDING APPARATUS". With this type of thermal recording apparatus, if the information to be recorded is all white, no heating resistor element is energized while all the heating resistor elements are energized if the information to be recorded is all black. Since each heating resistor element is energized for a preset period of time, the drive power source must have a constant voltage characteristic in order that every heating resistor element may be supplied with a constant amount of power even if the number of the used heating resistor elements is changed. If the voltage of the drive power source is appreciably lowered when all the heating resistor elements are energized, the power supplied to each element is decreased so that the element cannot generate heat enough to color the thermally responsive recording paper clearly. Thus, the resultant recorded information has a poor contrast and is difficult to recognize. Especially, if the recording speed must be set higher, a constant voltage drive power source is needed which can supply a very high voltage or a very large current. In that case, the size of the entire apparatus must be correspondingly increased and moreover the production cost also becomes higher. In general, the chance of recording the entirely or almost black information is very small. In the ordinary documents, the black is very small in comparison with the white. It is therefore uneconomical to prepare the drive power supply apparatus for the all black recording.